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There is also a set of lecture notes I have sitting on my desk by him that claim they are "to be published in Particle Physics and Cosmology" which are dated 1988, so presumably they were published within the next year or so.

If you can find them, the talks are probably the easier of the three, and the first PrD article is the second easiest. The third paper is very nice, but more technically difficult.

Is the third one "Cosmological Perturbations from Quantum Fluctuations to Large Scale structure"? Because that's the "starred" reference in a Part III DAMTP lecture I'm attending, but the article simply doesn't seem to be online anywhere.

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The single best reference for learning about gauge-invariant cosmological perturbation theory is ch. 7 of Mukhanov's book Physical foundations of Cosmology. Ok, not an online resource, but still the very best if that's what you're looking for.

On doing a search for one of York's original papers I ran into this wonderful site by Luca Bombelli at Ole' Miss (University of Mississippi). This contains a very comprehensive bibliography on the initial-value problem. I would also recommend Robert Wald's GR book. It has very nice coverage of the IVP in chapters and in an appendix.

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